Tag: fluid dynamics

Seeing Shock Waves
This week NASA released the first-ever image of shock waves interacting between two supersonic aircraft. It’s a stunning effort, requiring a cutting-edge version of a century-old photographic technique and perfect coordination between three airplanes – the two supersonic Air Force T-38s and the NASA B-200 King Air that captured the image. The T-38s are flying in formation, roughly 30 ft apart, and the interaction of their shock waves is distinctly visible. The otherwise straight lines curve sharply near their intersections.
Fully capturing this kind of behavior in ground-based tests or in computer simulation is incredibly difficult, and engineers will no doubt be studying and comparing every one of these images with those smaller-scale counterparts. NASA developed this system as part of their ongoing project for commercial supersonic technologies. (Image credit: NASA Armstrong; submitted by multiple readers)
March 8, 2019
How the Hagfish Deploys Its Slime
Hagfish – an eel-like species – are known for their prodigious slime production, which helps them escape predators (and, in some cases, seriously muck up highways). Part of the hagfish’s slime consists of ~10 cm fibers that the creature deploys in tiny skeins (bottom) only a hundred microns across. To form the viscoelastic slime that thwarts its predators, those skeins of fiber have to unravel and do so in only tenths of a second. A new study shows that viscous drag plays a major role in that unraveling.
Most fish use a suction method to catch prey. In the hagfish’s case, that does the predator more harm than good because the very flow it creates to try and catch the hagfish pulls the slime skein apart and helps the slime expand 10,000 times in volume, creating a mess that chokes the gills of the attacking fish. (Image credit: top – L. Böni et al.; bottom – G. Choudhary et al., source; research credit: G. Choudhary et al.; via Ars Technica; submitted by Kam Yung Soh)
March 7, 2019
Viscoelasticity and Liquid Armor
One proposed method for improving bulletproof armor is adding a layer of non-Newtonian fluid that can help absorb and dissipate the kinetic energy of impact. Thus far researchers have focused on shear-thickening fluids – like cornstarch-based oobleck – filled with particles that jam together if anything tries to deform them quickly. But is it really the shear-thickening properties that matter for high-speed impacts?
To test this, researchers studied projectile impact on three fluids: water (left), a cornstarch mixture (not shown), and a shear-thinning polymer mixture (right). Water is Newtonian, and it slows down the projectile but doesn’t stop it. Both the shear-thickening cornstarch and the shear-thinning polymer mixture do stop the projectile. And by modeling the impacts, researchers concluded that the key to that energy dissipation isn’t their shear-related behaviors: it’s the fact that both fluids are viscoelastic.
That means that these fluids show both viscous (fluid-like) and elastic (solid-like) responses depending on the timescale of an impact. The high speed of the impact triggered a strong viscous response in both fluids, bringing the projectile to a halt. And if, as the researchers suggest, it’s a fluid’s viscoelasticity that matters most, that widens the field of candidates when it comes to developing a fluid-based armor. (Image and research credit: T. de Goede et al.)
March 6, 2019
Sorting Blood Cells
Many diseases – like sickle-cell anemia and malaria – are accompanied by changes in the stiffness of red blood cells. And while microfluidic devices capable of sorting blood cells by size exist, few have made microfluidic devices capable of sorting blood cells by their deformability. But a new set of simulations suggests we could do so relatively easily.
Existing devices sort blood cells by size using an array of tiny posts – kind of like a cellular pachinko machine. Through simulation, researchers found that by changing the shape of these posts – specifically by turning them from circles into sharper triangles – they could sort the red blood cells by their stiffness. Because the sharp corners create large local stresses in the fluid, the blood cells get deformed when passing the corner. That ends up deflecting stiffer cells into a different stream. Build a whole array of posts and you can sort the blood cells by their degree of stiffness – ideally allowing you to isolate the most diseased cells. (Image and research credit: Z. Zhang et al.; via APS Physics)
ETA: Added a clarification: some researchers, like Beech et al., have investigated deformability-based sorting devices.
March 5, 2019
Melting
File this one under “Oddly Satisfying” – this timelapse video shows the process of melting a jawbreaker candy using a blowtorch. Over a minute and a half, each colorful layer of candy melts away to reveal the strata beneath. There’s a definite connection here to some of the previous research we’ve discussed on erosion, dissolution, and melting. The blowtorch’s flame generates a hot boundary layer around the candy surface; it’s thickest and hottest at the central stagnation point, but judging by the melting layer we see running all the way to the candy’s shoulder, its size and effect are substantial even there. It’s hard to tell from the video whether the surface of candy is getting roughened (a la scalloping) or whether that’s just an uneven layer of melted candy flow. Regardless, it’s a fun watch. (Video and image credit: Let’s Melt This; via Colossal)
March 4, 2019
Noisy Jets
One major problem that has plagued supersonic aircraft is their noise. The Concorde – thus far the only supersonic commercial airliner – was plagued with noise complaints that ultimately restricted its usability. Noise reduction is a major area of inquiry in aerospace, and the video below shows one experiment trying to understand the connections between supersonic flow and noise.
Above you see a supersonic, Mach 1.5 microjet emanating from a nozzle at the top of the image. The jet is hitting a flat plate at the bottom of the image. Just beyond nozzle’s exit, you can see the X-shape of shock waves inside the jet. The position of that X is oscillating up and down.
In the background, you can see horizontal light and dark lines traveling up and down. Those horizontal lines in the background are acoustic waves. When they hit the bottom plate, they reflect and travel upward until they hit another surface (outside the picture) and reflect back down. As they travel, they interact with the jet, causing those X-shaped shock waves to move up and down. This coupling between flow and acoustic waves makes the jet much louder – up to 140 dB – than it would be otherwise.
Researchers hope that unraveling the physics of simpler systems like this one will help them quiet more complicated aircraft. (Image and video credit: F. Zigunov et al.)
March 1, 2019
Landslide Lubrication
In 2008, an 8.2 magnitude earthquake in China caused the enormous Daguangbao landslide, which loosed over one cubic kilometer of rocks and debris. That material rushed down the mountainside, running more than 4 kilometers before coming to a stop. A new study uses field measurements and laboratory experiments to explain how the landslide could run so far from its source.
The researchers found that friction between the sliding material and the stable rock heated that layer to over 850 degrees Celsius, hot enough to start decomposing the dolomite in the fall. That vaporized carbon dioxide out of the rock, which helped lower the friction. Simultaneously, the high temperatures and high pressures within in the landslide caused recrystallization in the falling rocks; this created a viscous layer that helped lubricate the slide. The team estimated that the two mechanisms working in tandem enabled the landslide to reach an estimated 60 m/s. (Image and research credit: W. Hu et al.; via Nature; submitted by Kam-Yung Soh)
February 28, 2019
“The World Below”
Since the first cosmonauts and astronauts entered orbit around our planet, they’ve held a unique perspective. Thanks to the timelapse photography of recent astronauts aboard the ISS and the editing skills of photographer Bruce W. Berry, Jr, the rest of us can enjoy a taste of that viewpoint. Turn up the volume, fire up the big screen, and enjoy.
I particularly like how several of the sequences show off the depth of the atmosphere. Earth’s atmosphere is incredibly thin compared to the size of our planet – less than one percent of Earth’s radius – but thanks to the shadows that clouds cast on one another, you can really appreciate their height in sequences like the one at 2:26. (Video credit: B. Berry, Jr. using NASA footage)
February 27, 2019
Bats in Ground Effect
As pilots can tell you, flying near the ground (or an open expanse of water) gives one an aerodynamic boost. Essentially, the surface acts like a mirror, reflecting and dissipating the wingtip vortices that create downwash. That reduces the power necessary to fly, as long as you’re flying within about a wingspan of the surface.
Theoretically, flapping fliers like bats and birds should also benefit from this ground effect, but measurements have been hard to come by. A new study using bats trained to fly in a wind tunnel provides some of the first detailed measurements of ground effect for flapping animals. The researchers found a 29% reduction in the power necessary for flight when in ground effect compared to being out of it! That’s twice the savings predicted by modeling, meaning we still have a ways to go to accurately capture the physics of flapping flight under these circumstances.
Such a substantial savings also strengthens arguments for flight developing from the ground up. Using ground effect, surface-dwelling animals could have evolved flight gradually, taking advantage of the energy savings offered by sticking close to the surface. (Image and research credit: L. Johansson et al.; submitted by Marc A.)
February 26, 2019
Inside a Wind Tunnel
When I was in graduate school, I worked in a facility known as the High-Speed Wind Tunnel Lab. We were located next door to the Low-Speed Wind Tunnel, and every few months we’d receive a phone call asking whether we could film someone in the high-speed wind tunnel. This was impossible for several reasons – the size of human beings and the necessity of drawing the hypersonic tunnels down to vacuum-like pressures before initiating flow being only two of them – but what it really did was highlight the difference in definitions.
What these (usually) weather forecasters wanted was to simulate hurricane force winds on a human being. And to an aerodynamicist, that hundred mile-an-hour flow is still low-speed. Because we’re comparing it to the speed of sound, not the normal range of wind speeds a human experiences. That said, watching humans struggle inside a wind tunnel is always entertaining.
As you can see from the Slow Mo Guys here, counteracting the lift and drag forces from these wind speeds is tough. On the bottom left, Dan has managed to balance his weight and the drag forces to hold himself in a virtual chair. Meanwhile, Gav’s attempt to jump forward against the wind just pushes him backward as his lab coat parachutes behind him. (Image and video credit: The Slow Mo Guys)
February 25, 2019